Power field effect transistors (FETs) are increasingly used to drive high inductive loads. In an example application, a power FET drives a headlamp or headlight in an automobile or other vehicle. The power FET is coupled between the power supply and a high current bulb using a cable to couple current from the power FET circuit to the headlamp. The cable and the bulb can present a high inductive load in excess of 5 μH. In an automotive example application, the inductive load can be in a range between 5-20 μH. The bulbs of some headlights presently in use require a very high current at turn on, in an example the “inrush” current (the current drawn at the initial turn on) for the headlight can be in the range of 90-100 Amperes.
The power FET has a safe operating area (SOA) of current and voltage that can be maintained without damage to the device. The SOA is determined by the manufacturer of the FET. The SOA indicates the current and voltage conditions that need to be maintained to ensure reliable operation without damage to the FET and to ensure that the FET will meet an expected lifetime of the product. In order to protect the power FET and keep the device operating within the recommended SOA, current limiting is utilized. However, if a very high current through the FET occurs that causes a current limiting circuit to shut down or to limit the current flowing in the FET device, the energy stored in the inductive load has to be discharged. Although the FET may be off or may be turning off, the FET is still in the circuit between the inductive load and the power supply, and the stored energy is discharged through the FET. The discharge of the inductor energy through the FET can cause undesirable device stress or can even destroy the FET because the SOA boundary conditions can be violated. A specific characteristic of a conventional current limit or shutdown of a power FET circuit is that there can be current overshoot at turn off or current limit of the FET that exceeds the boundary of the SOA for the FET.